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.. ' . <br />Mr. William S, Tate -3- December 30, 1981 <br />4. Sediment Ponds <br />The method employed by Ute Engineering for pond designs is a simplified <br />triangular hydrograph methodology. The calculations appear to be complete, <br />except for dewatering device design. A value for gpo (peak overflow) <br />was obtained which would be required to provide a settling time of 24 <br />hours. However, this information was not utilized during dewatering device <br />design. As such, the proposed dewatering devices will have much higher <br />peak outflows and much shorter detention times. In order to provide the <br />29-hour detention times desired, the following procedure is recommended: <br />a. Determine the maximum head on the orifice for the dewatering <br />device during storage of the 10-year, 24-hour storm, <br />b. Assume that the control will be orifice A=(A (2gH))0'SC0.6, and <br />c. Solve for area A. <br />This approach is somewhat simplistic, but will provide designs for de- <br />watering orifice sizes which are appropriate for pond detention time <br />requirements. These orifices will undoubtedly be less than 2 inches in <br />diameter. The applicant has expressed concern that orifices less than <br />2 inches in diameter may tend to clog. This concern is mitigated, how- <br />ever, by the fact that all ponds have primary, as well as emergency, <br />spillways. ' <br />5. Emergency Spillways <br />The method used for design of emergency spillways at the site was an <br />application of Manninq's equation. As discussed under item 3 of this <br />letter, use of this method when using CMP's is inappropriate. The applicant <br />should utilize the enclosed chart in order to verify the adequacy of the <br />proposed spillways and submit the revised spillway design calculations <br />to the Division. <br />If you have any questions concerning these review comments, please do not <br />hesitate to call either Mr. Richard Domingue or myself. <br />Sincerely, <br />" ~~~ ~ ~ <br />Tom Cillis <br />Reclamation Specialist <br />TC/mt <br />Enclosure <br />cc: Mr. Floyd Johnson, OSM <br />